The present invention relates to ultrasonics, and more particularly to all or portions of an ultrasonic system which includes coupling signals between transducer and an object or body of material through which those signals are to propagate. Particular applications may involve coupling into a gas at high temperature where some form of isolation, such as a buffer rod, is required.
Buffer rods have been used in ultrasonics for over fifty years to separate a transducer crystal from media under investigation that are at very high temperature. This is analogous to tending a red-hot fire with a long steel poker so as to not burn one""s hands.
In ultrasonic measurements, it is important that the buffer not corrupt the signals of interest. Accordingly, much effort over the years has gone into avoiding the sidewall echoes generated by mode conversion. Such mode conversion occurs when longitudinal waves strike the wall near grazing incidence, generating shear waves, which in turn reflect multiple times diagonally across the rod. Each reflection generates a delayed replica of the original longitudinal pulse. Probably the most commonly-used way to avoid sidewall echoes is to thread the buffer. This method is often adequate, but has the disadvantage that it is quite lossy. For example, a solid steel buffer rod of 25 mm diameter and length of about 30 cm experiences a beam spread loss of about 20 dB, for a 500 kHz signal, assuming the longitudinal wave starts as a plane wave across the entire diameter of the rod.
Other known methods for preventing beam spread or diffraction loss in solid buffer rods include
(i) the use of shear waves rather than longitudinal waves, as described in U.S. Pat. No. 3,477,278 of L. Lynnworth, and
(ii) the use of a buffer rod in which the outer portion has a higher sound speed than the core, as described in U.S. Pat. No. 5,241,287 of Jen.
Furthermore, if hollow buffers are considered, one can avoid most of the diffraction loss and also avoid sidewall echoes. But in certain applications, it is not easy to correctly eliminate errors due to uncertainties in the time of travel down a hollow tube, to correctly eliminate errors due to uncertainties in the time of travel down a hollow tube, in which the sound-conducting fluid may have a temperature or compositional gradient. Hollow tubes also run the risk of becoming filled with residues or condensate, which loads the walls and significantly alters their propagation properties. Also, at high flow velocities they resonate and create strong acoustic interferences.
In 1966, I. L. Gelles described non-dispersive operation of individual or bundled glass fibers to form flexible ultrasonic delay lines. These bundles, formed of fiber optic cable encased in a loose plastic sheath, had their fibers all fused or joined together by epoxy to form a practical termination. However, Gelles found high attenuation even in a very short distance of propagation through the bonded region, reporting an attenuation of xe2x80x9croughly greater than 10 dB/mm of coated-fiber length.xe2x80x9d (I. L. Gelles, Optical-Fiber Ultrasonic Delay Lines, J Acoust. Soc. Am., 39 (6), pp. 1111-1119 (1966)). With this construction, Gelles was able to adjust the bond thickness to work better for a particular frequency or to minimize an undesired or spurious pulse, and he suggested that that construction would have usefulness for pick-off points, re-entrant delay lines and particular devices such as fiber-based acoustic modulators. However, to applicant""s knowledge, the acoustical use of optical fibers has not found application to transmission link or buffer constructions in the subsequent decades.
Thus, there remain problems in delivering or recovering well-defined acoustic signals when the process or measurement environment requires that the transducer be spatially remote. These problems may be particularly daunting when the process involves a gas at high temperature and high pressure, so that multiple considerations of physical isolation or containment, signal strength, and acoustic path impedance discontinuities all affect performance. For example, the seemingly simple requirement of measuring gas flow velocity accurately within xc2x11% over a wide flow range, for a low molecular weight gas, starting near or at atmospheric pressure and building up to 200 bar continuous, and for gas temperatures ranging up to 200xc2x0 C. in normal operation and 450xc2x0 C. in upset conditions, actually imposes a long list of requirements on the ultrasonic measuring system.
A practical system must accommodate considerations of cost, flexibility, signal isolation, and useful frequency range, as well as factors relating to calibration, maintainability and compatibility with existing equipment.
Accordingly, it would be desirable to provide an improved ultrasonic system.
In accordance with the present invention, a rigid link formed of stiff substantially non-dispersive rods is provided in an ultrasonic signal path. In one embodiment, the link is a buffer which acts as a signal-preserving and physically extending stand-off to isolate a transducer or coupling from destructive thermal, chemical or other physical conditions. Two or more such links may connect in series to carry signals between a transducer and an ultimate measurement or signal point. In a preferred embodiment, the rods are joined together in a solid sheet or disc at one end, which may be machined to a particular thickness or otherwise worked to augment or to enhance definition and coupling of the signal of interest. In a preferred embodiment of this aspect of the invention, the termination is a portion of a flange or cover plate forming a readily installed pressure-hardy closure for a process line or vessel. Each of the rods has a defined length so that they couple to transmit a combined signal or receive a detected signal coherently.
In other aspects or embodiments, the disc may include a xcex/4 matching layer for a specific frequency, and the link may operate in systems employing signals that are odd integral multiples of that frequency, or harmonically-related frequencies. In still other embodiments, the ends of a bundle may be terminated by coupling at different points along a surface to convert between compressional wave energy in the rods and flexural wave energy of the surface. The surface may for example be the wall of a cylinder acting as a high-frequency sound source or receiver, wherein the flexural excitation of the cylinder provides an impedance-matched coupling of the signal with a surrounding gas. The signal path link of the present invention propagates a given frequency at one phase velocity but may propagate different frequencies with different speeds. One system utilizing this property involves Fourier synthesis of special impulse waveforms built up from tone bursts of two or more different frequencies launched at different times from one end of the link. A single channel system utilizes such a link to provide a signal of enhanced edge definition. A pattern of different diameter rods in a bundle focuses or steers the beam, while shaping of the output face controls beam direction. A processing system may couple pairs of elements to cancel noise.